1. Field of the Invention
The present invention relates to a method for transcutaneously measuring access blood flow. More specifically, the invention relates to a method for measuring access blood flow through the optical measurement of percentage change in a blood parameter and application of the Ficke dilution principle.
2. Related Art
Modern medical practice utilizes a number of procedures and indicators to assess a patient""s condition especially in the dialysis setting. Hemodialysis is a process wherein an artificial kidney is required to function in the place of the patient""s normal kidney in order to remove certain biologic waste products. When the human kidney no longer functions correctly removing waste products such as urea, potassium, and even excess water, blood must be removed from the patient via blood tubing lines and filtered through an artificial kidney or dialyzer. In this process blood is passed through the dialyzer, cleansed, then returned to the normal circulatory system of the patient. Access to the patient""s circulatory system is achieved through the use of a surgically implanted shunt or fistula (access). This xe2x80x9caccess sitexe2x80x9d is typically located in the arm, leg, or neck of the patient. Typically needles are placed into the access in such a way as to facilitate the easy removal of blood on the xe2x80x9carterialxe2x80x9d or upstream side of the dialyzer and typically return the purified blood downstream of the first needle placement on the xe2x80x9cvenousxe2x80x9d side. Unfortunately, in many cases the access will clot or xe2x80x9cstenosxe2x80x9d over time. This results in decreased blood flow through the access site which ultimately necessitates either angioplasty or a surgical replacement of the shunt. As the access flow ceases or xe2x80x9cclots offxe2x80x9d part of the purified dialyzed blood is forced to flow back into the arterial withdrawal site and, hence, recirculates only to be dialyzed again; this is termed xe2x80x9caccess recirculationxe2x80x9d.
Access Blood Flow (ABF, represented by the variable Qa) is the rate at which blood passes through an arteriovenous (AV) graft or fistula. Poor or low Qa rates are generally indicative of hemo-dynamically significant access stenosis and/or thrombosis, which can reduce the adequacy of dialysis therapy and endanger the patient. In 1997 Dialysis Outcomes Quality Initiative (DOQI) Guidelines, the National Kidney Foundation (NKF) sets forth both the rationale and the procedural guidelines for the monitoring and maintenance of AV grafts and fistulas. These guidelines suggest that regular assessment of ABF may be predictive of access stenosis, which in turn may facilitate early intervention, thereby reducing the rate of thrombosis and loss.
NKF-DOQI Guidelines clearly identify access blood flow as a preferred method of monitoring AV grafts and fistulas: xe2x80x9cSequential, timely, repetitive measurement of access flow is the preferred method for monitoring AV graftsxe2x80x9d, and xe2x80x9cFlow measurements should be used when available to monitor for stenosis and thrombosis in AV fistulae.xe2x80x9d NKF-DOQI Pocket Summary, Clinical Practice Guidelines for Vascular Access: Guideline 10,11.
Lindsay and Leypoldt state, xe2x80x9cReductions in access blood flow rates if recognized may mandate reductions in QB and lead to difficulty in delivering adequate dialysis; if unrecognized these reductions can lead to the phenomenon of access recirculation, which will significantly decrease the efficiency of the hemodialysis treatment. Furthermore, such reductions may herald the problem of acute access thrombosis. It seems ideal, therefore, to monitor access blood flow.xe2x80x9d Lindsay R, Leypoldt J: Monitoring Vascular Access Flow. Advances In Renal Replacement Therapy, Vol. 6, No. 3 (July), 1999: pp. 273-277.
Blood flow, Q, measured by the so-called Ficke dilutional techniques, has been described by A. C. Guyton, Textbook of Medical Physiology, Sixth Edition, pg. 287, 1981, wherein Q equals the volume of the injected diluent divided by the mean concentration of the diluent times the duration of the passage of the diluent through the vessel. A dilution curve is obtained by continuously monitoring changes in a given physical parameter of the blood over the time period of the injection. The change in the concentration of either the diluent (or the media) is measured over time.
Access Blood Flow (ABF) measurement is an area of concern in hemodialysis since it is a good indicator of access viability. Recent methods of determining ABF have included Doppler imaging, reversed line recirculation, and             Δ      ⁢              xe2x80x83            ⁢      H        H    ⁢            (              the        ⁢                  xe2x80x83                ⁢        percentage        ⁢                  xe2x80x83                ⁢        change        ⁢                  xe2x80x83                ⁢        in        ⁢                  xe2x80x83                ⁢        hematocrit        ⁢                  xe2x80x83                ⁢        through        ⁢                  xe2x80x83                ⁢        the        ⁢                  xe2x80x83                ⁢        access        ⁢                  xe2x80x83                ⁢        site            )        .  
The time, cost, and/or dialysis line reversal requirements of these methods have greatly limited their wide spread use and routine clinical applicability. With the exception of Doppler, ABF methods require the patient to be on dialysis and unencumbered by intradialytic activity such as blood pressure assessment or eating, further reducing flexibility in measurement. Conversely, Doppler measurements remain limited in accuracy due to uncertainty in measuring access size and cross-sectional area.
It is to the solution of these and other problems that the present invention is directed.
It is therefore a primary object of the present invention to provide a straightforward method of determining ABF, Qa, using an optical sensor placed on the skin directly over the access site.
It is another object of the present invention to provide a transcutaneous method of ABF measurement that is not affected by size and/or depth of the access site, placement of the dialysis needles, pump speed variations, skin color, tissue composition, or access site location and type.
It is another object of the invention to provide a method of measuring a parameter transcutaneously downstream of a site where an indicator diluent is injected.
It is another object of the invention to provide a method of measuring a parameter transcutaneously in a perturbed system downstream of a site where the perturbation is introduced.
These and other objects of the invention are achieved by use of indicator dilution techniques to measure vascular access flow rates during routine hemodialysis, as well as in a clinic, before and/or after hemodialysis. A bolus injection port is used to infuse a specific volume (Vi) of an indicator diluent, such as saline or dye, into the patient cardiovascular circuit by one of the following:
1. Needle injection of a known volume (bolus) of indicator diluent directly into the access site in the presence or absence of the hemodialysis circuit.
2. Infusion of an indicator diluent into the arterial or venous needle or line upstream of the detector.
3. Turning the ultrafiltration of the dialysis delivery system from OFF to ON and OFF again over a predetermined time period.
4. In a hemodialysis circuit, turning on the hemodialysis pump and using the priming saline volume as a single saline bolus.
A transdermal optical sensor is used to measure the percent change in a blood parameter. The sensor is positioned directly over the vascular access site a prescribed distance downstream of the injection site and upstream of the access-vein connection in the case of grafts. The sensor employs complementary emitter and detector elements at multiple spacings (d1, d2) for the purpose of measuring the bulk absorptivity (xcex1) of the area immediately surrounding and including the access site, and the absorptivity (xcex1o) of the tissue itself.
In one aspect of the invention, the optical sensor system comprises an LED of specific wavelength and a complementary photodetector. A wavelength of 805 nm-880 nm, which is near the known isobestic wavelength for hemoglobin, is used.
When the sensor is placed on the surface of the skin, the LED illuminates a volume of tissue, and a small fraction of the light absorbed and back-scattered by the media is detected by the photodetector. The illuminated volume as seen by the photodetector can be visualized as an isointensity ellipsoid, as individual photons of light are continuously scattered and absorbed by the media. Because a wavelength of 805 nm-880 nm is used, hemoglobin of the blood within the tissue volume is the principal absorbing substance. The scattering and absorbing characteristics are mathematically expressed in terms of a bulk attenuation coefficient (xcex1) that is specific to the illuminated media. The amount of light detected by the photodetector is proportional via a modified Beer""s law formula to the instantaneous net xcex1 value of the media.
When the volume of tissue illuminated includes all or even part of the access, the resultant xcex1 value includes information about both the surrounding tissue and the access itself. In order to resolve the signal due to blood flowing within the access from that due to the surrounding tissues, the sensor system illuminates adjacent tissue regions on either side of the access. Values of xcex1o for tissue regions not containing the access are then used to normalize the signal, thus providing a baseline from which can be assessed in access hematocrit in the access blood flowing directly under the skin.
In the case that hematocrit is the monitored parameter, these values are then related to the percentage change of the parameter by the relationship:             F      ⁢              (                              Δ            ⁢                          xe2x80x83                        ⁢            H                    H                )              =                            d          ⁢                      xe2x80x83                    ⁢          i                i                              (                      d            -                          1              α                                )                ⁢                  (                                    α              2                        -                          α              o              2                                )                      ,
where i is defined from a modified Beer""s law as:
I≈IoAexe2x88x92xcex1d,
where A≈xcex1
More specifically, the diluent bolus is injected into the access site at an average flow rate of Qi. Since the hematocrit of the indicator solution is zero, the red blood cell (RBC) mass does not change. The transcutaneous access blood flow (TQa) differential equation of state may be re-written in either a transient formulation,             Q      a        =                  V        i                    ∫                              F            ⁡                          (                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  H                                H                            )                                ⁢                      ⅆ            t                                ,            where      ⁢              xe2x80x83            ⁢                        Δ          ⁢                      xe2x80x83                    ⁢          H                H              =          function      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      time      
or time dependent or steady flow formulation. The steady flow form of the TQa differential equation is obtained by assuming uniform and steady flow rates over the analysis time period, and is written as       Q    a    =            Q      i              F      ⁢              (                              Δ            ⁢                          xe2x80x83                        ⁢            H                    H                )            
In both cases, the quantity   F  ⁡      (                  Δ        ⁢                  xe2x80x83                ⁢        H            H        )  
is measured as the indicator bolus is injected into the system. If the bolus injection rate Qi is uniform and constant, then the access flow Qa may be determined from the steady flow formulation. Conversely, if the system remains dynamic and the bolus injection rate is uncertain or uncontrollable, then the transient solution must be used to determine Qa.
The percentage change in blood parameters (both macroscopic and microscopic) passing through the access site can be measured in a variety of ways. Macroscopic parameters such as bulk density or flow energy can be measured by ultrasonic, temperature, or conductivity means. Microscopic parameters (sometimes called xe2x80x9cphysiologic or intrinsicxe2x80x9d parameters) such as hematocrit or red cell oxygen content are measured by optical means. In both cases, the measurement relies on the quantity   di  i
when saline is injected. Thus, the method in accordance with the present invention can also be applied to the measurement of macroscopic parameters (percent change in density, temperature, conductivity, or energy) using ultrasonic, temperature, or conductivity sensors; and to the measurement of microscopic parameters like hematocrit using an optical sensor.
In the measurement of both macroscopic and microscopic blood parameters, it is necessary to differentiate the access site, and parameter changes therein, from the surrounding tissue structure. The method in accordance with the present invention utilizes a transdermal sensor incorporating photoemitters and photodetectors positioned directly over the access site itself and is based upon optical back-scattering of monochromatic light (xcex=805 nm-880 nm) from the blood flow in the access site and the surrounding tissues, so that it is not limited to the extracorporeal circuit.
Light back-scattered from a turbid tissue sample follows the modified form of Beer""s Law,
I≈IoAexe2x88x92xcex1d, A≈xcex1
A transcutaneously measured xcex1 value is a prorated composite measure of all the absorption and scattering elements contained within the illuminated volume or xe2x80x9cglowballxe2x80x9d of the emitter source, and typically includes the effects of tissue, water, bone, blood, and in the case of hemodialysis patients, the access site. The effects of absorption and scattering of the access site are separated from that of surrounding tissue structure by taking measurements in areas near but not including the access site. If the tissue is more or less homogeneous, it is only necessary to make a single, non-access site reference xcex1o measurement. On the other hand, if a gradient in xcex1o exists in the area of interest, multiple measurements are made to establish the nature of the gradient and provide an averaged estimate of xcex1o.
The value of       Δ    ⁢          xe2x80x83        ⁢    H    H
is defined as the time derivative of intensity i, normalized by i. To determine       di    i    ,
a baseline intensity (taken in the absence of a bolus) is first measured to establish a reference. The intensity is then measured as a time varying signal. The quantity   di  i
is then calculated as             d      ⁢              xe2x80x83            ⁢      i        i    =                    I        baseline            -              I        ⁢                  (          t          )                            I      baseline      
The value   F  ⁡      (                  Δ        ⁢                  xe2x80x83                ⁢        H            H        )  
consequently is:       F    ⁢          (                        Δ          ⁢                      xe2x80x83                    ⁢          H                H            )        =                              d          ⁢                      xe2x80x83                    ⁢          i                i            ⁢      α                      (                  d          -                      1            α                          )            ⁢              (                              α            2                    -                      α            o            2                          )            
Since d is fixed and known,       di    i    ,
xcex1 and xcex1o are computed by the equations:                     d        ⁢                  xe2x80x83                ⁢        i            i        =                            I          baseline                -                  I          ⁡                      (            t            )                                      I        baseline              and      α    ≈                  -                  Ln          ⁡                      (                                          I                measured                                            I                o                                      )                              d      
or from:
I≈Ioxcex1exe2x88x92xcex1d,
where xcex1 is solved in polynomial form.
where access size and/or volume or depth dependence are not factors in either the transient or the steady state formulation of Qa.
It is another object of the invention to provide a method of measuring a parameter transcutaneously in a perturbed system downstream of a site where the perturbation is introduced.
Although one embodiment of the invention uses saline as a diluent and measures the dilution of an existing endogenous material such as blood, the method in accordance with the present invention generally contemplates measuring a parameter transcutaneously in a perturbed system downstream of a site where the perturbation is introduced. The parameter can, for example, comprise a marker and the method comprises measuring the marker downstream using a sensor. Possible markers are proteins or red cells tagged with a radio nucleotide, which can be measured using a Geiger counter. Other parameters that can be measured in accordance with the present invention, and the devices for measuring them are: ultrasound, measured by an ultrasound detector; temperature, measured by a thermistor; impedance, measured by a bio-impedance measuring device; and albumen, glucose, and other blood constituents, measured using the optical sensor disclosed herein, but in which the LEDs emit different wavelengths suited to the specific constituent.
Other objects, features and advantages of the present invention will be apparent to those skilled in the art upon a reading of this specification including the accompanying drawings.